Conventionally, an electrophotographic process used in a copying apparatus or a printer is superior to the other image forming processes in a high speed, a high quality of an image and a capability of recording on plain paper, so this process has been generally and widely in common use.
Meanwhile, in recent years, a digital technique of the electrophotographic process has made progress and it is attracting attention. This technique is developed so that a copying apparatus can perform functions which cannot be performed by an analog copying apparatus. The functions are such that the digital copying apparatus can be connected to a facsimile or a computer, can edit and process an image.
In addition, as to a facsimile, thermal recording system using a thermal head and thermal paper is dominant in these days because of its low price and small size. However, since the thermal recording does not provide service life and stability of recording paper, the aforementioned conventional electrophotographic process which is capable of recording on plain paper is used for a certain purpose. Moreover, a facsimile printer which adopts the aforementioned digital technique into the thermal recording system has been developed. A schematic drawing of this apparatus is shown in FIG. 21.
The above apparatus includes a printer section which is an image forming section and a controlling section 318 for controlling input/output of information. The printer section have an information input section 311 composed of an image sensor such as CCD, a photoreceptor 312, a charging section 313 for uniformly charging the photoreceptor 312, an exposing section 314 for writing information to the photoreceptor 312, a developing section 315 for developing an electrostatic latent image, which has been formed on the photoreceptor 312 by exposure, using a developer, a transfer section 316 for transferring a developed image onto recording paper 319, and a cleaning section 317 for removing the developer which remains on a surface of the photoreceptor 312.
However, since in the conventional electrophotographic process, a corona charger is used for charging and transfer processes, ozone is generated. The ozone is harmful to not only humans but also service life of a photoreceptor, etc. Moreover, since the electrophotographic process is complicated, there exists a limit to miniaturization lowering of cost.
Therefore, as one of methods of solving these problems, a new recording process without corona charge is suggested in the Journal of Japan Society of Electrophotography, Volume 30, No. 3 (1991), p.323. This method makes it possible to simplify an image forming process and to miniaturize an apparatus by simultaneously carrying out charge, exposure and development. The following will describe this method.
As shown in FIG. 22, in the simultaneous charge-exposure-development (SCED) process, a photoreceptor 301 where a transparent conductive layer 301b and a photoconductive layer 301c are formed in this order on a transparent base 301a such as glass is used. In image forming, when a toner layer, which is composed of conductive magnetic toner (hereinafter, referred to as toner) ta, contacts with a surface of the photoreceptor 301, and a developing bias 303 is applied to an electrically conductive sleeve 302a of a developing roller 302, the toner layer has a potential which is same as the developing bias 303. In other words, in a position where the photoconductive layer 301c begins to contact with the toner layer, a surface potential of the photoconductive layer 301c is lower than the developing bias 303. Thereafter, charges are injected into the photoconductive layer 301c through the toner ta which adheres to a surface of the photoconductive layer 301c. Moreover, toner ta, which successively carried by rotation of a magnetic roller 302b of the developing roller 302, collide with the toner ta adhering to the photoconductive layer 301c, and the adhering toner ta leaves from the photoconductive layer 301c. Due to repetition of such movements, the surface potential of the photoconductive layer 301c becomes substantially same as the developing bias 303.
Therefore, even in a position where the toner layer lies apart from the photoconductive layer 301c, the surface of the photoconductive layer 301 has a substantially same potential as the developing bias 303. For this reason, the toner ta in the above position is attracted to the developing roller 302 side by magnetic force of the magnetic roller 302b. Meanwhile, when a light 304 is irradiated to the toner ta so that the photoreceptor 301 is exposed through the transparent base 301a just before the toner ta leaves from the photoconductive layer 301c, photo-exited carriers generated in the photoconductive layer 301c move to the surface of the photoconductive layer 301c and are neutralized with the surface charges. The surface potential of the photoconductive layer 301c is lowered due to the neutralization, and the charged toner ta adheres to the photoconductive layer 301c so as to cover the lowered potential. Since the toner ta adhering to the photoconductive layer 301c immediately leaves from the developing roller 302 in this position, there is not time for charging the photoconductive layer 301c through the toner ta. Therefore, in the above position, electric attraction becomes stronger than magnetic attraction by the magnetic roller 302b, thereby making it possible to obtain a visible image of toner on the surface of the photoreceptor 301.
A concrete apparatus adopting the aforementioned SCED process is disclosed Japanese Laid-Open Patent Publication 4-138767/1992. As shown in FIGS. 23 and 24, this apparatus is provided with a print head 332 and a image sensor 334 in a transparent photoreceptor drum 331 composed of a transparent electrode and a photoconductive layer. The print head 332 uses a light emitting diode or a laser, and the image sensor 334 consists of the light emitting diode to irradiate a light on a document 333 and a photoconductive device to read its reflected light.
In addition, in order to achieve space saving, Japanese Laid-Open Patent Publication 2-83559 discloses that a printer adopting the SCED process is provided with an aperture-type light source which is used as an exposing section and with functions in exposing and eliminating charges. As shown in FIG. 25, its arrangement is such that a fluorescent lamp 341 is used as the light source, a liquid crystal array panel 342 and a focusing lens 343 are used as the exposing section and that charges are eliminated from a photoreceptor 345 through an aperture 344. Moreover, in order to increase light availability, a movable reflecting plate 346 is provided on a surface of the fluorescent lamp 341.
In addition, Japanese Laid-Open Patent Publication 1-196076/1989 discloses an apparatus where as a developer, photoconductive toner is used in the SCED process instead of conventional dry toner. As shown in FIG. 26, in this apparatus, not a conventional photoreceptor but an image carrier 351 where a transparent dielectric layer is formed on a transparent base is used and an image is formed by photoconductive magnetic toner 352.
Furthermore, Japanese Laid-Open Patent Publication 4-190369/1992 discloses coloring of a formed image in the SCED process which uses photoconductive toner. As shown in FIGS. 27(a) and 27(b), its arrangement is such that (1) an image carrier 361 where a transparent conductive layer and a photoconductive layer are formed on a transparent carrier, (2) a white light source 362, (3) a shutter for forming image 363, (4) filters for selecting wavelength 364y, 364m, 364c and 364b, (5) respective photoconductive toner for yellow, magenta, cyan and black, and (6) developer vessels 365y, 365m, 365c and 365b which individually contain the photoconductive toner for each color are provided, and that a color image is obtained by allowing an exposing section 366 to rotate until the exposing section 366 comes towards the filters for selecting wavelength 364y, 364m, 364c and 364b to be used.
An image forming process using the photoconductive toner is described in 9th International Congress on Advances in Non-Impact Print Technologies/Japan Hardcopy 1993, p.189. In this process, photoconductive toner composed of zinc oxide showing persistent photoconductivity, and instead of the SCED process, similarly to the conventional electrophotographic process, exposure is performed from an outside of an image carrier. The above process will be explained referring to FIG. 28. Photoconductive toners, which are negatively charged by mixing with carrier beads in a development unit 372, are uniformly deposited on a surface of a metal drum 371 as a thin layer and a light is irradiated thereon by an exposing section 373. As a result, an exposed portion of the toner layer gradually discharges charges due to absorption of the light and becomes low resistance states, but an unexposed portion maintains a initial charges. Here, when a negative bias is applied to a transfer roller 374, an effective charge injection occurs from a metal drum 371 to the exposed portion. With this injection, the polarity of toner charge converts to the opposite sign so that the exposed portion is transferred onto a sheet 375. In such a manner, an image is formed.
In the above-mentioned image forming process using the photoconductive toner, dry photoconductive toner is used, but a wet-system image forming process where photoconductive toner is dispersed in insulating liquid has been conventionally known. This process is described in, for example, J.Appl. Photo. Eng. 8 (1982), p.256. However, this process is not an SCED process either. The process will be explained referring to FIG. 29. A thin coating layer of photoconductive toner is applied to a Mylar belt 382 by an ink unit 381, and the photoconductive toner is negatively charged by a corotron 383 as pre-charging. Next, the photoconductive toner layer is carried to a metal drum 384. Here, positive charges are applied to the photoconductive toner by a corotron 385 on an inner side of the Mylar belt 382 and at the same time that the photoconductive toner is exposed by a laser head 386. As to the exposed photoconductive toner, its polarity is reversed by charge injection due to absorption of the light so that the photoconductive toner moves towards the metal drum 384. A negative image is obtained on the metal drum 384 and a positive image on the Mylar belt 382. Thereafter, the toner image on the metal drum 84 is transferred onto paper 387.
As mentioned above, in the image forming apparatus adopting the SCED process, since a corona charger is not used for charging and transferring process, ozone is not generated. Moreover, since an image forming process is simple and an exposing unit is positioned on an opposite side of a developing unit side to an image carrier for forming an image of a developer, space can be effectively utilized and an apparatus can be miniaturized. This becomes particularly remarkable in the case where a drum-like or endless belt-like image carrier is used because the exposing unit can be positioned inside the drum-like or the endless belt-like image carrier.
However, as to the arrangements shown in FIG. 23 or FIG. 24, since a print head 332 and an image sensor 334 for exposure are provided inside a transparent photoreceptor drum 331, an apparatus is miniaturized, but the print head 332 and the image sensor 334 for exposure are isolated respectively, so a light source is required for each. For this reason, miniaturization of an apparatus is not sufficiently attained.
In addition, in the arrangement shown in FIG. 25, although the exposing section has functions in exposing the photoreceptor 345 and eliminating charges from the photoreceptor 345, an arrangement having a function in reading a document image is not considered. Moreover, miniaturization of an apparatus is not sufficiently considered. Furthermore, since the aperture for exposure of the photoreceptor 345 and the aperture 344 for charge eliminating of the photoreceptor 345 are mechanically opened and shut by the movable reflecting plate 346, it is not easy to control driving of the movable reflecting plate 346, and also perfect shading in the above apertures is difficult. Moreover, since the movable reflecting plate 346 requires a mechanical driving structure, a size of an apparatus becomes large.
In addition, in the arrangement shown in FIG. 26, miniaturization of an apparatus is not considered. Moreover, in the arrangement shown in FIGS. 27(a) and 27(b), similarly to the arrangement shown in FIG. 25, an arrangement having a function in reading a document image is not considered, and also miniaturization of an apparatus is not sufficiently considered. Here, since a color image is obtained by rotating the exposed portion 366, it is necessary to precisely control a position of the exposed portion 366, and this control is difficult. Moreover, a driving mechanism for rotating the exposed portion 366 is required, so a size of an apparatus becomes large.
Needless to say, since the arrangements shown in FIGS. 28 and 29 are not a process using the SCED process, miniaturization of an apparatus cannot be desired.
As mentioned above, with the above-mentioned conventional arrangements, there arises a problem that miniaturization of an image forming apparatus cannot be sufficiently attained.